This invention relates to processes for reordering, i.e., increasing the moisture content, and drying tobacco. More particularly, this invention relates to the use of controlled humidity air to moisten or dry tobacco.
The tobacco art has long recognized the desirability of controlling the moisture content of tobacco. The moisture content of tobacco that has been processed into a useful product has been altered numerous times. Each processing step, e.g., stem removal, cutting, blending components, adding flavors, expansion and fabricating into cigarettes, requires certain optimum moisture levels, which must be controlled carefully, to ensure top quality tobacco products. Moreover, the manner in which the moisture content of the tobacco is altered can have a lasting effect on the physical, chemical and subjective characteristics of the final product. Accordingly, the methods used for bringing about changes in the moisture content of the tobacco are important.
Reordering of expanded tobacco is a particularly demanding process. Typically, tobacco obtained from the expansion process will have a moisture content below 6%, and often less than 3%. At such low moisture contents the tobacco is very susceptible to breakage. Additionally, the expanded tobacco structure is subject to collapse upon reordering, i.e., a full or partial return of the tobacco to its unexpanded state. This collapse results in a loss of filling power, thus decreasing the benefit derived from the expansion process.
Various means for reordering expanded tobacco have been used. The most common method is to subject the tobacco to a water spray, typically while tumbling the tobacco in a rotating cylinder. Another method is to use saturated steam as the reordering medium. Yet another method is to blow high humidity air through a moving bed of tobacco on a conveyor, as shown in U.S. Pat. No. 4,178,946.
None of the above methods has been found to be completely satisfactory for use on expanded tobacco. Tumbling tobacco in a spray cylinder results in breakage of the fragile expanded tobacco. Direct contact with liquid water tends to cause collapse of the expanded tobacco structure. Steam reordering also results in expanded tobacco structure collapse. While this may be partially attributed to the high temperatures in a steam environment, exposing expanded tobacco to any gaseous environment in which water condensation occurs, such as a steam or highly humidified air environment, results in collapse.
One method, which has been employed to avoid these difficulties, is to place dry, expanded tobacco in a chamber containing air at a desired humidity level and allow the tobacco to equilibrate in the chamber over a period of from 24 hours to 48 hours. Air velocity through the chamber is kept very low, typically not more than about 25 feet per minute. This procedure results in little or no collapse of the expanded tobacco structure. However, the long times required, 24 hours to 48 hours, have limited its application to laboratory purposes.
Attempts have been made to reduce the residence time required of such equilibration processes by increasing air velocity. Such approaches have been unsuccessful due to an inability to duplicate the maintenance of filling power observed in slow laboratory equilibration, the size of conveyors required to carry the tobacco in order to accommodate the long residence times required, the nonuniformity of the moisture content of the tobacco product exiting such conveyors, and the incidence of fires in such units as described in U.S. Pat. No. 4,202,357.
The use of drying as a means for controlling moisture content during the processing of tobacco is of equal importance as that of reordering. When tobacco is dried, both physical and chemical changes can occur that affect the physical and subjective quality of the product. Therefore, the method of drying tobacco is exceedingly important.
There are two types of drying equipment generally used by the tobacco industry: rotary driers and belt or apron driers. Pneumatic-type driers are also used occasionally. The particular dryer used is chosen for the drying operation required. Belt or apron driers, for example, are normally used for strip tobacco, whereas rotary driers are used for cut tobacco. Both rotary and belt driers are used for drying stems.
In a belt dryer, tobacco is spread on a perforated belt and air is directed either upward or downward through the belt and tobacco bed. Nonuniform drying of the tobacco often occurs due to channels being blown in the bed allowing the drying air to locally bypass the tobacco.
Most rotary driers used in the tobacco industry are lined with steam coils and may function as either indirect or direct heat driers depending on whether the heat is applied outside or inside the drier shell containing the tobacco. Moreover, they may be operated either co-currently where the tobacco and air flow in the same direction or countercurrently where the tobacco and air flow in opposite directions. Rotary drying must be controlled carefully to avoid overdrying, which causes both chemical changes and unnecessary breakage by the rotary motion. In addition, if drying occurs too quickly, an impervious layer may be formed on the outer surface of the tobacco making it difficult for moisture on the inside of the tobacco to diffuse to the surface. The formation of such a layer slows the drying rate and results in nonuniformity in drying.
Use of a rotary or belt drier to dry tobacco can result in a thermal treatment that may result in chemical and physical changes to the tobacco. While not always undesirable, these changes are driven by the objective of removing water from the tobacco. In typical tobacco applications, the need to dry the tobacco in a limited amount of time dictates a thermal treatment result from the drying step, preventing optimization of thermal treatment apart from the process constraints imposed by drying.
The present invention provides a means of reordering or drying tobacco with little or no breakage, even of the fragile tobacco exiting the expansion process. It further provides a means of reordering expanded tobacco with little or no loss of expanded tobacco structure. It further provides a means of drying tobacco at approximately atmospheric pressure, for example, without the use of vacuum and at a selected temperature wherein the thermal treatment imparted can be controlled during the process to an extent unattainable in conventional tobacco drying processes.